The present invention relates to a detection circuit for detecting the supply voltage value of a single power supply at two levels.
In an arithmetic processing unit which uses a solar battery as a supply voltage source, a malfunction may occur because the solar battery has a small current-carrying capacity and lacks the capacity to supply a constant voltage. Conventionally, for the purpose of preventing occurrence of a malfunction in the arithmetic processing unit, two supply voltage detection circuits are provided for detecting the voltage at two different levels, and the supply voltage is monitored such that a malfunction does not occur in the arithmetic processing unit even when the supply voltage varies. In the case where the supply voltage decreases to a level such that a malfunction may occur, the operation of the arithmetic processing unit is stopped so that the arithmetic processing unit is set in a sleep state. Thereafter, when the supply voltage is recovered, the arithmetic processing unit is switched to the active state to resume its operation. As for an arithmetic processing unit which involves an operation period where arithmetic processing is not performed, the operation state of the arithmetic processing unit is switched between the active state and the sleep state according to the operation status of the arithmetic processing unit, thereby reducing the current consumption by an entire LSI device. The above technique is described in, for example, Japanese Unexamined Patent Publication No. 58-168122.
However, the two supply voltage detection circuits monitor the supply voltage all the time and therefore consume the electric current all the time. In the case where the arithmetic processing unit is set in the sleep state according to detection results of the supply voltage detection circuits such that the current consumption is reduced, the current consumption by the entire LSI device decreases, and accordingly, the proportion of the current consumption by the supply voltage detection circuits to the current consumption by the entire LSI device increases. Thus, the current consumption by the supply voltage detection circuit cannot be ignored.